Temperature sensors incorporating thermistors whose resistance changes with temperature are commonly used. Thermistors have either a positive or a negative temperature coefficient. A positive temperature coefficient means that the resistance of the thermistor increases with increased temperature. A negative coefficient means the thermistor resistance decreases with increasing temperature.
In a typical prior art configuration, the thermistor is series coupled with a known resistance to form a voltage divider. A known voltage is applied across the voltage divider and the voltage at an output junction between the known resistance and the thermistor is measured to determine the thermistor temperature. The temperature is derived from the output junction voltage since that voltage varies with thermistor resistance which is directly related to the thermistor temperature.
Thermistors are calibrated by their manufacturers and a table of thermistor resistances at different temperatures are sent to the customer with each thermistor. Generally the tabulated thermistor resistances are accurate within 10-20 percent at a given temperature. Very accurately calibrated "precision" thermistors are available at substantially increased cost. If precision thermistors are matched with resistors whose resistance is known to a high degree of accuracy (i.e., "precision" resistors) a precision voltage divider can be constructed.
Resistors can be adjusted or trimmed extremely accurately to produce "precision" resistors. Trimming can be accomplished in various ways. For example, the resistance of wire wound resistors is trimmed by adjusting the number of windings that make up the resistor. Thick film resistors deposited on substrates are sometimes trimmed by adjusting the locations of conductive connector fingers along the resistor (see U.S. Pat. No. 4,085,399) and sometimes by physically removing thick film resistance material until a desired precise resistance value is achieved.
Highly accurate electronic temperature sensors have, in the past, required the use of precision thermistors and precision resistors coupled together in voltage divider or wheatstone bridge networks. Because of the high initial cost of components and because careful matching of components having precisely determined values was required to produce sensor networks having characteristics which were consistent from network to network, electronic temperature sensor networks of the character referred to have not been available in large quantities at low cost.
Furthermore, many of the prior art temperature sensors were so constructed and arranged that mechanical stresses tended to be transferred to the thermistors during use. The applied stresses altered the thermistor resistance characteristics, frequently decalibrating the temperature sensor networks.